1. Technical Field of the Invention
The present invention relates to an induction machine system including therein a wound-rotor induction machine for controlling the secondary current vector to perform a secondary excitation control.
2. Prior Art
In the wind-power generation, for the purpose of converting a wind energy to an electric energy so that the wind energy becomes maximum with respect to changes of a wind velocity or force, adjustable-speed running corresponding to the wind velocity is conducted. Moreover, in the water-power generation, running is conducted at a rotation speed where the efficiency of a water turbine becomes maximum with respect to changes of water head or changes of the load, thereby making it possible to perform high efficiency running. In addition, in such a case that a generator is directly coupled to the shaft of a propeller of the ship to effect power generation, the adjustable-speed power generation is strongly required. For one of such adjustable-speed generation systems, there is known a system to control the secondary current of the wound-rotor induction machine using a frequency converter to effect a control such that the frequency of the primary side is kept constant even if the rotation speed changes, which is so called the "secondary excitation system". Because this system advantageously permits the capacity of the converter to be reduced, it is especially suitable for large capacity power generation plant.
The voltage control apparatus based on the secondary excitation system of the wound-rotor induction machine of this kind has been already proposed (See FIGS. 3. 2. 11, page 96, research paper BMFT-FB-T84-154(1), by Bundesministerium fuer Forschung und Technologie in West Germany). In this apparatus, the primary winding of the wound-rotor induction machine is connected to the system. The frequency converter comprised of a cycloconverter is connected to the secondary winding, thereby conducting the secondary current control. Namely, three-phase command values of the secondary current are delivered to the frequency converter. These command values are compared with detection values of the secondary current, thus to effect a control such that the detection values are always equal to the command values. In addition, the primary current is detected with it being divided or separated into the current component in phase with the primary voltage and the current component lagging it by a phase angle of 90 degrees. By the product of these detection values and the absolute value of the primary voltages, the effective power on the primary side and the reactive power on the primary side are detected. On the other hand, command values of the effective and reactive powers are given. Then, these command values are compared with respective detection values, whereby the secondary current vector is controlled through the effective power adjuster, the reactive power adjuster and the frequency converter so that the compared deviations are equal to zero.
As just described above, the conventional system has been the system to control the secondary current vector of the wound-rotor induction machine in accordance with the command values of the effective and reactive powers. Accordingly, the primary side of the wound-rotor induction machine apparently serves as a current source, so that the primary voltage follows the changes of the system voltage. For this reason, in the event that the system voltage is disturbed due to the system trouble, the conventional system lacks the ability of maintaining the system voltage constant. Particularly, in such a case that one line is cut off due to the one line grounding trouble and is opened, there was an inconvenience such that the primary voltage is greatly distorted because of the configuration of the control system, so that commutation failure happens in the cycloconverter constituting the frequency converter, or an overvoltage occurs. Accordingly, the conventional system has found limited use of extremely small capacity power generation plants subsidiary to the system, which only impregnate a generated power into the system. Therefore, the conventional system was not applicable to the large capacity power plants having the duty to maintain the system voltage.